Integrated circuit fabrication processes include steps in which materials must be selectively removed with respect to other materials. The removal process may use either wet or dry etching. Regardless of the method selected, are selectivity of the etch between the materials that are removed and those that are not removed should be high. However, selectivity is not the only consideration, the skilled artisan readily appreciates that consistent results over an extended period of time should be achieved.
Dielectric regions, termed field oxides, are formed in integrated circuit fabrication by depositing a dielectric mask layer over silicon (likely having a thin oxide, termed a pad oxide on the surface), pattering the dielectric mask layer to expose the silicon or oxide, and growing a thick oxide in the exposed region. The thick oxide is termed the field oxide. A typical choice of dielectric for the mask layer is silicon nitride. This process is frequently referred to as LOCOS which is a well known acronym for Localized Oxidation of Silicon. In current practice, a polysilicon layer is frequently present underneath the nitride layer. The patterned structure is referred to as a Poly Buffered LOCOS (PBL) stack. Thus, both silicon nitride and polysilicon must be removed without significant attack on the silicon oxide.
Wet etching techniques are commonly used for blanket stripping of nitride and silicon in the presence of oxides. Silicon nitride are etched in boiling phosphoric acid. Minor amounts of nitric acid may be present in the etch bath. These baths are susceptible to aging effect and high particle counts because the silicate concentration in the bath increase with wafer throughput. Other approaches use separate baths for the nitride and polysilicon. This not only increases the number of baths required but may also lead to pitting of the pad oxide.
Of course, etching of these materials in other contexts is also contemplated. See, Solid State Technology, Ernst Gaulhofer, for a description of etching apparatus and etchants. It is reported that nitric, hydrofluoric, and phosphoric acids are used in a 3:1:1 solution to etch silicon. No selectivity is reported nor is there any discussion of rejuvenating the etch solution which is important to minimize chemical usage.
These problems were generally overcome by using a bath that had phosphoric, hydrofluoric, and nitric acids present. See, U.S. Pat. No. 5,310,457 issued on May 10, 1994 to David Ziger which is incorporated herein by reference. The bath life is extended by the addition of hydrofluoric and nitric acids. The bath had a relatively constant etch rate as a function of time. This feature is attributed to a reaction between the hydrofluoric acid and the etch products which leads to reaction products that do not interfere with the rates of the etch chemistry. It is believed that the nitric acid is required to oxidize the silicon.
However, it has been found that the initial etching selectivity of nitride to oxide may be undesirably low, selectivities of 9:1 have been observed, and depends on the relative fluorine concentration in the etch bath. Higher initial etch selectivities are desirable.